There is known one conventional muffler (particularly sub-muffler) for an internal combustion engine as shown in FIG. 11, in which an outer pipe 102 is mounted around an outer periphery of an inner pipe 100 (which forms an exhaust gas passage), with a space 101 formed therebetween, and one end portion 103 of the outer pipe is fixedly secured to the inner pipe 100 through a buffer member 105 such as a wire mesh, and the other end portion 104 thereof is fixedly secured to the inner pipe 100.
In one known method of producing such a muffler, the one end portion 103 of the outer pipe 102 is first reduced in diameter by a press die 106 as indicated by broken lines in FIG. 12A, and then the inner pipe 100, having the buffer member 105 fitted on one end portion thereof, is inserted into the outer pipe 102 as shown in FIG. 12B, and the buffer member 105 is disposed at the inner peripheral surface of the contracted one end portion 103. Then, as shown in FIG. 12C, the other end portion 104 of the outer pipe 102 is reduced in diameter into a tapering shape by a press die 107, and is fixedly secured to the inner pipe 100.
In the above muffler, generally, the other end portion 108 of the inner pipe 100 remote from the buffer member 105 projects from the contracted other end portion 104 of the outer pipe 102, and serves as a welding portion, and the muffler is connected at this welding portion 108 to an exhaust pipe by welding.
In the above conventional production method, however, that portion of the outer pipe 102, having the buffer member 105, is first reduced in diameter, thereby connecting the inner and outer pipes 100 and 102 together, and therefore during the contraction of the end portion 104 of the outer pipe remote from the buffer member 105, which is effected thereafter, there is a possibility that the inner pipe 100 and the outer pipe 102 are displaced relative to each other in the axial direction.
Namely, the reasons for this are that the strength of frictional connection between the outer pipe 102 and the buffer member 105 is low, and that the outer pipe 102 is pushed in the axial direction by the press die 107.
If the inner and outer pipes are thus displaced relative to each other, a welding area L of the welding portion 108 to be welded to the exhaust pipe is reduced, which results in a problem that it becomes difficult to effect the welding.
Further, the muffler, in which the outer pipe 102 is reduced in diameter by the press die, has a problem that the buffer member 105 can not be compressed uniformly over the entire periphery thereof, and besides in the construction in which the opposite end portions of the outer pipe 102 are reduced in diameter by the press die, the material of the outer pipe can be buckled, and therefore the limit of the contraction rate is about 20%, which leads to a problem that there can not be produced the muffler in which the volume of the space, formed between the inner and outer pipes, is increased.